The present invention relates to a method of working a diffraction optical grating element used in an image output unit, particularly a color image output unit, in an image forming apparatus, e.g., a copying machine and a printer.
The present invention also relates to a method of working a mold for molding an optical element such as a diffraction grating element.
The present invention also relates to a method of forming a groove in an optical element, particularly a diffraction grating element shape.
An example of use of a diffraction grating element in an apparatus which line-scans a subject such as an original and reads and obtains the color image information of the subject with an image sensing element array is disclosed in, e.g., U.S. Pat. No. 5,838,480.
According to this reference, an image information beam from the subject is guided onto an image-forming substrate through an optical system on the first optical path and the first and second lens elements on the second optical path.
The first lens element has a diffraction grating element shape.
With this arrangement, the image information beam can be split into a plurality of beams having different wavelength ranges, and the plurality of beams can correctly form an image on the image-forming substrate without color misregistration.
Japanese Patent Laid-Open No. 8-336701 proposes a method of working a curved surface having an arbitrary shape with high surface precision.
FIG. 12 shows an optical scanning unit in a color image forming apparatus incorporating an optical element according to the present invention.
Referring to FIG. 12, reference numeral 1 denotes a light source means such as a semiconductor laser; 2, a collimator lens; 4, an aperture diaphragm; and 6, a cylindrical lens having a predetermined refracting power in only a subscanning direction to cause a beam passing through the aperture diaphragm 4 to form a linear image on the reflecting surface of an optical deflector 8 (to be described later) within a subscanning section.
Reference numeral 8 denotes a polygon mirror serving as the optical deflector and rotatably controlled by a driving means (not shown).
Reference numeral 10 denotes a scanning optical element with f"THgr" characteristics and having a refracting portion and a diffracting portion. A refracting portion 10A is a toric lens having different powers in the main scanning direction and subscanning direction. A diffracting portion 10B is comprised of an elongated diffraction optical grating element having different powers in the main scanning direction and subscanning direction.
Reference numeral 12 denotes a photosensitive drum.
Working of the optical element according to the present invention concerns the elongated diffraction optical grating element.
When seen from above, the diffraction grating used in the color image reading apparatus described above forms a combination of substantial elliptic shapes in the major- and minor-axes directions, as shown in FIG. 1.
The diffraction grating used in this example forms a plurality of concentric ellipses having fine grooves, as shown in FIGS. 3 to 5 with a section taken along the line Axe2x80x94A through the center in the minor-axis direction and a section taken along the line Bxe2x80x94B through the center in the major-axis direction.
The color image reading diffraction grating is formed on the flat surface of a rectangular parallelepiped metal substrate 1 shown in FIG. 2 by using this member 1, formed long in the major-axis direction, as the mold.
FIG. 4 shows a ridge line of the grooves near the center in the Axe2x80x94A direction.
FIG. 5 shows a ridge line of the grooves near the center in the Bxe2x80x94B direction.
The numerical values of the respective portions of the diffraction grating of this example are as follows.
The width of the rectangular parallelepiped 1: 9.648 mm
The length of the rectangular parallelepiped 1: 225.12mm
The material of the rectangular parallelepiped: phosphor bronze
A gap P among grooves: 0.729727 mm to 0.009882 mm
A height h1 of the inclined portions of the grooves: 0.001488 mm
A height h2 of the inclined portions of the grooves: 0.001488 mm
An inclination angle xcex11 of the grooves: 0.1168 degree
An inclination angle xcex12 of the grooves: 8.56 degree
Also, the number of grooves is 2,577.
Grooves must be formed in above number and with the above groove sizes within an area having a width W of 9.648 mm and a length of 225.12 mm of the rectangular parallelepiped.
Referring to FIG. 1, the grooves of the diffraction grating have substantially elliptic portions M in which the elliptic shapes formed by the grooves are completely closed, and substantially elliptic portions N1, . . . , and Nn in which the elliptic shapes formed by the grooves are not closed.
When the diffraction grating element shape described above is to be worked, the blade is moved on the workpiece in the X-, Y-, and Z-axis directions along the curves of the diffraction grating. Not only the blade is moved in the X-, Y-, and Z-axis directions, but is operated biaxially to match predetermined angles with respect to the X-, Y-, and Z-axes of the workpiece, thereby performing working.
The blade must be moved in the X-, Y-, and Z-axes of the workpiece, and its posture must be controlled biaxially.
To form a diffraction optical grating element shape on the upper working target surface of a workpiece W, as shown in FIG. 6, a curved surface R1 at the central portion of the diffraction grating element shape is formed on the upper surface of the workpiece W, and after that a second groove portion R2 is formed at the outer peripheral position of the curved surface R1 of the central portion. Second and third groove shapes are sequentially formed by cutting.
The concave curved surface at the central portion is formed in the following manner. As shown in FIG. 3, the edge of the blade is moved with a predetermined feed pitch in the X-axis direction to form moving traces K1, K2, K3, . . . shown in FIG. 3 with respect to the flat X-, Y-, and Z-axis coordinate planes on the working target surface of the workpiece W. In the Z-axis direction, the blade is controlled such that its cutting edge follows the shape of the curved surface of the central portion.
In this case, as shown in FIG. 6, part of the blade which forms a curved surface G1 at the central portion interferes with the worked curved surface, and a contact mark with the blade portion is formed on the worked curved surface. This impairs the optical diffraction function.
In order to solve the problems arising when working the diffraction grating element shape described above, according to the present invention, there is provided a method of working a diffraction optical grating element shape on a working target surface, characterized in that a main spindle for rotatably supporting a cutting blade is provided on X-, Y-, and Z-coordinate axes of the working target surface, and an angle of an edge of the cutting blade mounted on the main spindle performs working to have an inclination with such an angle that interference with a planned working position at an outer peripheral portion of a curved surface of the diffraction optical grating element is avoided.
According to the present invention, there is also provided a method of working a mold for molding a diffraction optical grating element shape, characterized in that a main spindle for rotatably supporting a cutting blade is provided on X-, Y-, and Z-coordinate axes of a working target surface of the mold, and an angle of an edge of the cutting blade mounted on the main spindle performs working to have an inclination with such an angle that interference with a planned working position at an outer peripheral portion of a curved surface of the diffraction optical grating element is avoided.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part hereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.